This invention relates to a process for removing sulfate from an aqueous salt solution by way of precipitation, and greatly simplifying separation of the sulfate free liquid from the precipitate.
The use of soluble salts to precipitate sulfate out of an aqueous salt solution is well-known in general chemistry. Common applications of this basic chemistry are found, for example, in the desulfurization of photographic wash water, as well as the desulfurization of liquid bittern, to name a couple. More specifically, the use of calcium chloride to precipitate sulfate out of liquid bittern is a well-known practice. Liquid bittern is the bitter water solution of mainly magnesium chloride and some residual sulfate, remaining after sodium chloride is crystallized out of seawater.
The removal of sulfate from aqueous salt solutions is often carried out for purposes of purification of the solution, or for more extensive purposes, as in the case of liquid bittern. Sulfate tends to precipitate out of liquid bittern when the temperature drops, causing handling problems during transportation of the bittern to cooler climates and storage in cold climates. A primary end use of liquid bittern is as a deicer, which inherently requires its use in cold climates. Removal of the sulfate from liquid bittern alleviates storage and handling problems associated with the formation of settleable sulfate solids.
In the process of removing sulfate from liquid bittern, calcium chloride is typically added rapidly to the liquid bittern, resulting in a very finely divided dispersion of calcium sulfate precipitate. This dispersion requires expensive and time-consuming processing, such as centrifugation or filtration, to remove the precipitate.
There is thus a need or desire for a simple, effective, low-cost method for removing sulfate from an aqueous salt solution.
The present invention is directed to a method of precipitating sulfate in an aqueous salt solution and removing the precipitate from the solution. The method includes diluting the aqueous salt solution and slowly adding a precipitant to the solution.
The precipitant is slowly added to the aqueous salt solution, either after the solution has been diluted or simultaneously while diluting the solution. The mixture can be agitated by bubbling with air, or agitated by other mechanical means, during the addition of the precipitant such that most of the precipitant added will encounter entrained seed crystals of a sulfate precipitate, favoring growth on the seed crystals and sulfate precipitate particles that are large enough to readily settle to the bottom of the reaction tank. After addition of the precipitant solution is complete, the reaction mixture is allowed to stand undisturbed for between about 8 and about 24 hours, during which time the sulfate precipitate settles essentially quantitatively to the bottom of the tank. A flocculating agent may be added to the reaction mixture before it is allowed to settle to improve the clarity of the supernatant. After settling, the supernatant sulfate-free solution can be pumped from the tank.
There is no limit to how much the solution could be diluted, but it should be diluted to a specific gravity upper limit of about 1.35 or lower. The precipitant addition rate can be varied widely depending on the size of the specific operation and the degree of mixing that is possible. There is no lower limit on how slowly the precipitant can be added. It is important that the addition be slow enough that seed crystals from the first amount of precipitant added are present and able to react with subsequent precipitant as it is added. The precipitant cannot be added instantaneously to the solution. Another embodiment of this invention includes seeding the solution with sulfate precipitate before addition of the precipitant.
A preferred precipitant is calcium chloride, although other precipitants can be used. When calcium chloride is used as the precipitant, the calcium chloride reacts with the sulfate in the solution to form insoluble calcium sulfate, which precipitates from the solution.
The slow addition of precipitant coupled with dilution of the starting solution produces a sulfate precipitate that settles out of solution quantitatively, greatly simplifying separation of the sulfate-free liquid from the precipitate.
With the foregoing in mind, it is a feature and advantage of the invention to provide a simple, effective, cost-efficient method of removing sulfate from an aqueous salt solution.
Sulfate can be removed from an aqueous salt solution through addition of a precipitant and subsequent precipitation of the sulfate. However, the ease with which the precipitated sulfate can be separated from the supernatant liquid is highly dependent upon the specific manner in which the mixing of the solution and precipitant is carried out. Using the method of the invention, an aqueous salt solution can be economically desulfated in a simple batch process. This invention can be used in many different applications, including in the desulfurization of photographic wash water, as well as the desulfurization of liquid bittern.
One of the steps of the invention includes diluting the solution. More specifically, the solution is diluted to a specific gravity of less than about 1.35, preferably less than about 1.33. There is no lower limit to how much the solution could be diluted, but it should be diluted to an upper limit of about 1.35 or lower. Alternatively, in terms of another measure of dilution applicable for when the solution is liquid bittern, the bittern can be diluted to less than about 34%, preferably less than about 32% magnesium chloride.
Another step in the invention includes slowly adding a precipitant to the solution. The precipitant addition rate can be varied greatly, depending on the size of the operation, the degree of mixing that is possible, and the concentration of the solution. For example, in a well-mixed system, a calcium chloride precipitant (on an anhydrous basis) can be added at a rate of between about 0.05 pounds per hour per cubic foot and about 300 pounds per hour per cubic foot, preferably the calcium chloride can be added at a rate of between about 0.2 pounds per hour per cubic foot and about 30 pounds per hour per cubic foot, more preferably the calcium chloride can be added at a rate of between about 0.2 pounds per hour per cubic foot and about 4 pounds per hour per cubic foot. The precipitant should not be added instantaneously to the solution. There is no lower limit on how slowly the precipitant can be added. It is important that the addition of the precipitant be slow enough that seed crystals from the first amount of precipitant added are present and able to react with subsequent precipitant as it is added. Generally, the more poorly mixed the solution is, the more slowly the precipitant should be added.
A preferred precipitant, particularly when liquid bittern is the solution, is calcium chloride. In one embodiment of the invention, the liquid biftern can be seeded with calcium sulfate before the addition of calcium chloride to the solution. In another embodiment, calcium chloride can be added to xe2x80x9cvirginxe2x80x9d bittern, before the bittern has been concentrated to saturation in magnesium chloride. Other suitable precipitants for sulfate include soluble salts of calcium, barium, strontium, or lead, as well as other known equivalents.
The combination of diluting the starting solution and slowly adding the precipitant produces a sulfate precipitate that settles out of solution quantitatively, greatly simplifying separation of the sulfate-free supernatant from the precipitate. It is desirable that both dilution of the solution and slow addition of the precipitant be carried out for this process to work. If either one is neglected, the sulfate precipitate will not settle out quantitatively. It is believed that this method works by causing the sulfate crystals to grow larger and decrease the density of the liquor, both of which facilitate settling of the solid. In general, it appears that the more dilute the solution and the slower the addition of the precipitant, the better the sulfate precipitate settles out.
The aqueous salt solution is preferably agitated by bubbling with air, or agitated by any other mechanical means, during the addition of the precipitant such that most of the precipitant added will encounter entrained seed crystals of the sulfate precipitate, favoring growth on the seed crystals and leading to sulfate particles that are large enough to readily settle to the bottom of the reaction tank. After addition of the precipitant is complete, the reaction mixture is allowed to stand undisturbed for between about 8 and about 24 hours, preferably between about 12 and about 24 hours, during which time the sulfate precipitate settles essentially quantitatively to the bottom of the tank. After settling, the sulfate-free supernatant can be pumped from the tank.
Alternatively, the precipitant can be added to the aqueous salt solution with no mixing, simply allowing the precipitant to gradually diffuse into the solution. Once the precipitant has diffused into the solution, the mixture can be stirred briefly, or agitated by any other mechanical means, and then allowed to settle, thereby yielding a clear, desulfated supernatant.
A flocculating agent can be added to the reaction mixture before it is allowed to settle in order to improve the clarity of the supernatant. Suitable flocculating agents include anionic polymers, more particularly, non-toxic polyacrylamide-polyacrylate copolymers. For example, a copolymer of sodium acrylate and acrylamide is an effective flocculating agent. An example of one such flocculating agent in the form of a commercial product is Aqua Process 7607 Polymer, available from Aqua Process, Inc.
Depending on the concentration of the solution and how quickly the precipitant is added to it, the supernatant liquid can have varying degrees of turbidity, and addition of an anionic flocculent is very effective in removing this turbidity. For example, calcium chloride solution was slowly added to a sample of 35% magnesium chloride bittern, resulting in a supernatant liquid that was cloudy and analyzed as 0.29% sulfate. Another sample prepared in the identical manner, except that 10 ppm of Aqua Process 7607 Polymer was added, had a very clear supernatant liquid that did not have any detectable sulfate in it.
However, if the calcium chloride is added slowly to a more dilute bittern solution, the calcium sulfate settles out so effectively that the supernatant liquid is quite clear even without use of a flocculating agent. For instance, when calcium chloride was added slowly to 31% magnesium chloride bittern, a fairly clear supernatant liquid resulted which was analyzed as containing only 0.18% sulfate.
Addition of 2.5-10 ppm flocculating agent, such as Aqua Process 7607 Polymer anionic flocculating polymer, either prior to or after addition of the precipitant, removes residual turbidity from the desulfated solution. Maximum effectiveness appears to occur at about 10 ppm addition of Aqua Process 7607 Polymer. Increasing the amount of Aqua Process 7607 Polymer to 100 ppm does not appear to increase the clarity of the supernatant liquid or increase the settling of the sulfate precipitate. Typically, less flocculent is needed in large batches.